Recently, demand on thinning layer, larger sizing and higher performance for the polarizing plate protecting film to be used in a display such as a liquid crystal display rise accompanied with development of thin and light note type personal computers and large screen televisions. In such the situation, the narrow viewing angle has considered hitherto as a problem. For making a wider viewing angle, it is proposed to provide a function of optical compensation sheet to the protecting film, (for example, refer to Japanese Patent Publication Open to Public Inspection (hereafter referred to as JP-A) No. 6-222213).
Moreover, a technique of addition of a crosslinking agent into the protective film for preventing tear of the polarizing plate is proposed (for example, refer to JP-A No. 2001-55402). The yield of the polarizing plate can be raised by such the technique.
Furthermore, an addition of a rod-shaped compound to the protective film is proposed for solving a problem of variation of the displayed image such as the contrast, tone and gradation, particularly variation in tone, depending on the viewing angle (for example, refer to JP-A Nos. 2002-267847 and 2004-4550).
The variation of the displayed image in the contrast, tone and gradation depending on the variation of viewing angle and the occurrence of tear of the polarizing plate can be improved by the above-described means but the effects of them have not been fully sufficient. Furthermore, a problem is posed that the retardation value of the protective film used in the usual polarizing plate is reversely varied accompanied with variation of the humidity. Such the phenomenon relates to variation in the quality of displayed image on the liquid crystal display. Therefore, a technical means for dissolving such the problem has been desired.
Furthermore, it has been found that the light leaking caused by thermal distortion cannot be completely prevented when the polarizing plate using an optical compensation sheet is installed as the protecting film on a large size panel of not less than 17 inches. It is necessary that the optical compensation sheet not only has the function of optically compensating the liquid crystal cell but also has excellent durability against variation of the environmental condition.